It is essential for human activity that pipelines be laid, in order to transport gases, (usually natural gas), liquids (water, crude or refined hydrocarbons) and even slurries (e.g. coal, etc., in a liquid carrier). Commonly, these pipelines are buried, for protection against surface traffic, etc. After some years, surface vegetation or weathering may effectively mask the location of the pipelines. In some instances, there may be no information about the precise path of a pipeline, either due to the fact that its path was never accurately surveyed, or that the documentation about its path has since been lost. This may be true even if the pipeline is still being actively used for its original purpose.
In the event of further activity in the vicinity, which may necessitate the disturbance of the surface, it becomes a vital matter to precisely determine the location of the pipelines in the area, in order to avoid the possibility of inadvertent rupture of a pipeline. The consequences, in terms of personal injury or financial loss, of the rupture of a gas or oil pipeline, for example, could be very serious.
A common occurrence, for example, is the need to conduct further seismic tests, utilizing new technology, in the vicinity of a producing oil field. Seismic tests require the detonation of explosive charges, or the use of heavy vibrators, whose sites must be well clear of existing pipelines.
It may also occur, in the case of an oilfield or network of water wells where a series of wells feeds a system of collection by pipes, that the precise location of the individual wells is no longer known but must be determined, to facilitate further activity in the field.
Although means are known for locating buried pipes by ground measurements, such means usually rely on the ability to contact the pipe at one known point and, in any case, are slow and labour intensive if a large area, encompassing multiple pipelines, is to be mapped.
Pipelines are commonly made of steel, as an inexpensive, strong and durable material. Where the metal of the pipe comes into direct contact with groundwater, however, corrosion may occur, due to electrochemical action. To help guard against such corrosion, pipelines are usually covered with an impervious coating before burial. In addition, the electrical potential of the pipe is often lowered relative to the ground by use of a suitable DC generator, thus providing "cathodic protection" against corrosion, but causing DC current to flow along the pipe.
Wellheads are usually characterized by having rather long lengths of near-vertical steel casing, in order to maintain the integrity of the well and to allow gases or fluids to be brought to the surface.
Other ferrous objects, such as buried drums, submerged vessels, buried ordnance, etc. may themselves be the targets of interest for detection and precise positioning. These are usually much more limited, dimensionally, than pipelines or wellhead castings.